1. Technical Field
This application is generally related to measurement and control of electric motors, for example, multi-pole permanent magnet machines.
2. Description of the Related Art
An electric motor is an apparatus or machine for producing motion and mechanical effects by the action of electricity. Those having ordinary skill in the art typically recognize three standard or classical motor designations: direct current (“DC”) motors with commutators (wound field), synchronous alternating current (“AC”) motors and asynchronous AC motors.
One example of synchronous AC motors is the permanent magnet AC synchronous motor. As shown below, the permanent magnet motor is a synchronous motor, and thus the stator frequency and the number of magnetic poles may be used to directly determine motor speed.
As with most motors, the permanent magnet synchronous motor has two primary parts. The mechanically non-moving or stationary part is called the stator, and the mechanically moving or rotating part, usually inside the stator, is called the rotor.
In order to enable a motor to rotate, two fluxes are needed: one from the stator and the other from the rotor. Typically, in an electric motor, at least one flux is generated in the stator which is of opposite polarity and proximate to at least one flux generated in the rotor. Although the stator is mechanically stationary, electromagnetic techniques may be utilized to create a flux which rotates internal to the stator. Although the rotor can move mechanically, the flux of the rotor is often stationary internal to the rotor. Hence, the moving flux internal to the stator cause the rotor to mechanically rotate by attracting the opposite polarity flux which is stationary internal to the rotor.
There are at least two ways to generate rotor flux. One way is to use windings integral with the rotor to generate an electromagnetic field. Another way is to use permanent magnets integral with the rotor which generate a magnetic field, and hence flux.
One common, commercially available, motor is the “three phase” motor, which uses three alternating current waveforms, slightly delayed in time relative to each other to generate the moving flux in the rotor.
As noted, AC electromagnetic techniques are typically utilized in order to generate the at least one moving flux in the stator, and permanent magnet techniques may be utilized to generate the stationary flux in the rotor. One way in which this is done is to construct the stator as an electromagnet made with a winding for each phase of the motor, while permanent magnets may be used for generating rotor flux. In each winding of the stator, current may either flow in a forward or positive direction, or in a reverse or negative direction. As current flows in the forward direction in the stator according to its windings, the rotor is forced to rotate as it tries to align itself with the electromagnetic flux. At a certain point in the rotation, the field is reversed and the rotor continues to turn in an attempt to realign itself with the new, or negative, field orientation. For a three-phase motor, this results in six unique steps or pole alignments. The amount of current flow may be controlled by either pulse width modulation or analog means. The resolution of control actually depends upon the resolution of the positioning feedback device, the current feedback, and the update rate.
As noted, motors typically operate by “dragging,” or pulling the stationary fluxes in the rotor with the moving fluxes induced in the stator. Those having ordinary skill in the art will appreciate that in order to do the foregoing effectively, the moving fluxes in the stator are generally controlled to keep those fluxes effectively proximate to the opposite-polarity fluxes in the rotor. Those having ordinary skill in the art will appreciate that in order to effectively do the foregoing, it is helpful to have a good idea of either or both the speed of rotation and positioning of the fluxes inside the rotor.
One way in which either or both the speed of rotation and positioning of the fluxes inside the rotor are determined is via the use of sensors that detect the poles of the magnets stationary within the rotor. For example, multi-pole brushless permanent magnet machines use magnetic pole position sensors, such as Hall effect sensors, to determine the positions of the magnetic poles within the rotor, relative to a fixed position on the stator, so as to control current in the stator winding in order to provide a controlled torque output (e.g., reversing direction of current every half cycle). As used herein, the output of a magnetic pole position sensor constitutes one logic level when it is near a north pole and another logic level when it is near a south pole. Thus, as the poles of the rotor pass near the magnetic pole position sensor, the logic level output alternates from low to high. For a machine with N pole pairs (or N×2 poles), there will be N sequences of low and high logic levels on the output of the magnetic pole position sensor for each mechanical revolution of the stator of the machine.
In this type of system, speed is measured by determining the time difference between the edges of the output signal of the magnetic pole position sensor. This can be done in a variety of ways. The most common way is to measure the time from one rising edge to the next. This time is then multiplied by the number of poles of the machine and inverted giving the rotational speed of the motor. This concept is graphically illustrated with respect to FIG. 2.
FIG. 2 shows an electric motor 200 in cross-section. The electric motor 200 includes a rotor 202 having four poles provided by permanent magnet. Alignment lines 204 illustrate the alignment of the north poles and the alignment of the south poles such that the N-S-N-S poles are equispaced around the rotor 202. Broken directional lines 206 show the manipulation of the currents in the windings (not shown) of stator 210 such that rotating S-N-S-N poles are produced in the stator. Broken directional lines 208 illustrate that the rotating S-N-S-N poles in stator 210 drag their respectively paired N-S-N-S poles in rotor 202 to cause rotation of rotor 202.
A number of rotor-pole sensors 212 (e.g., hall effect sensors) are positioned or proximate the on stator 210. Those skilled in the art will appreciate that, insofar as the N-S-N-S poles in rotor 202 are equispaced, it is common in the art to measure the elapsed time of rotation between successive N-S poles, and then multiply the measured elapsed time of rotation by the number of poles in the rotor to determine the speed of rotation of the rotor. For example, using one of the rotor-pole sensors to detect the elapsed time between successive poles, and thereafter multiplying the measured elapsed time by four, since there are four poles which divide the rotation into 4 equal length arcs.
As has been explained above, in order to provide effective control of a motor, it is important to have an accurate measure of the speed of the motor. It is therefore apparent that a need exists in the art for the motor measurement speed to be accurate.